1. Field of the Invention
The present invention relates to a digital deflection control system in a projection TV and, more particularly, to a digital deflection control system in a projection TV, which digitally interpolates the horizontal deflection signal for three red, green and blue color signals which are color image signals.
2. Discussion of Related Art
In a projection TV, picture light emitted from its optical projection unit is obliquely projected to the screen since its optical system is generally arranged unsymmetrically to the screen. This brings about distortion in images projected to the screen. To correct this, the picture light is intentionally distorted before it is projected to cancel the distortion caused by the optical system. This process is generally called matching correction. While the matching correction is performed by employing an analog or digital approach, there has been recently proposed and used a method, one of the digital approach, which controls the horizontal and vertical deflection signals themselves supplied from the deflection system.
FIG. 1 shows a block diagram of a conventional digital deflection control system in a projection TV. Referring to FIG. 1, the digital deflection control system of the conventional projection TV includes an electrically erasable programmable read only memory (EEPROM) 10 which stores seed data for generating horizontal and vertical deflection signals for red, green and blue (R, G, B) colors, a central process unit (CPU) 20 which loads the seed data for R, G, B stored in EEPROM 10 when the system is reset and then carries out first interpolation for the seed data (this interpolated data is called subseed data), a digital convergence system (DCS) 30 which secondary-interpolates the subseed data to generates digital horizontal and vertical deflection signals, and an analog circuit 70 which converts the digital horizontal and vertical deflection data outputted from DCS 30 into analog signals having corresponding magnitudes, amplifies them up to a predetermined level and applies them to horizontal and vertical deflection coils.
Analog circuit 70 consists of three D/A converters 40R, 40G and 40B which convert the subseed data for R, G, B outputted from DCS 30 into analog step waves having corresponding magnitudes, three analog switches 50R, 50G and 50B which are switched by horizontal and vertical output control signals HH and VH outputted from DCS 30 to output horizontal deflection signals RH, GH and BH and vertical deflection signals RV, RG and RB for R, G, B in the form of step wave, and six LPFs and amplifiers 60RH, 60GH, 60BH, 60RV, 60GV and 60BV which smooth horizontal and vertical deflection signals RH, GH, BH, RV, GV and BV in step waveform sent from analog switches 50R, 50G and 50B to waveform-shape them and then amplify them. The LPFs are conventionally configured of fifth-degree butterworth filters.
With the conventional digital deflection control system of the projection TV, CPU 20 performs first interpolation for the R, G, B horizontal and vertical seed data stored in EEPROM 10, for example, 5*5 data, to obtain 16*5 subseed data, and DCS 30 carries out secondary interpolation for the 16*5 subseed data to create 16*256 point data required for actual image representation, generating the horizontal and vertical deflection signals. Here, 16 and 256 mean horizontal and vertical deflection points, respectively. The point data created as above then passes through D/A converters 40R, 40G and 40B, analog switches 5R, 50G and 50B, and LPFs/amplifies 60RH, 60GH, 60BH, 60RV, 60GV and 60GV to be applied to the horizontal and vertical deflection coils.
As described above, however, the conventional digital deflection control system uses the D/A converters, analog switches and fifth-order LPF filters and amplifiers in order to convert the sixteen pieces of point data for generating the horizontal deflection signal into the analog signals, resulting in high production cost and large sized system.